Band-pass filter for hyperfrequencies

ABSTRACT

This highly reproducible stripline band-pass filter comprises, between its two leads (A and B), n halfwave resonators (1-7) connected in series. Suitable capacitors (C0-C7) provide a powerful coupling between the successive elements of the series circuit. Quarterwave resonators (10, 11, 70, 71) each having one end connected to the series circuit provide a steep-edge amplitude/frequency response at the limits of the pass-band of the hyperfrequency, wideband filter so obtained.

This invention concerns band-pass filters for electromagnetic wavesbelonging to the UHF and microwave ranges, i.e. included in the"hyperfrequency" category as defined by the InternationalElectrotechnical Commission (IEC), and in particular concerns widebandfilters and filters constructed with stipline waveguides.

The technique is known of constructing microwave band-pass filters, forexample in the technological field of stripline transmission, by seriesconnecting a low-pass filter and a high-pass filter; the low-pass filterconsists of a series of narrow line sections and of wide line sectionsserving respectively as the inductive and capacitive elements of thefilter; the high-pass filter consists of narrow line sections which aregrounded and which serve as inductive elements and are connectedtogether by open-circuited lines or capacitors. Such filters arerequired to have a large number of poles in the low-pass filter and inthe high-pass filter, are required to include a large number of linesections. They are therefore bulky and costly.

Another type of such filters in the prior art consists of a series ofresonators disposed between the entrance and the exit of the filter,which are powerfully coupled together; the resonators are placed veryclose to one another to obtain a powerful coupling. This type of filteris difficult to manufacture, even in stripline or microstrip form, ifthe desired coupling entails that any two successive resonators bespaced less than 100 microns apart, because this coupling must beperfectly constant from filter to filter to ensure consistentcharacteristics for all filters in a given production run.

It is the object of this invention to obviate, or at least abate, theabove-mentioned disadvantages.

The improvement of the invention basically resides in a resonator-typefilter in which on the one hand the coupling of successive resonators isreinforced by means of suitably disposed capacitors and on the otherhand a band-stop capability is introduced.

The invention provides a band-pass filter for hyperfrequencyelectromagnetic waves, comprising, all connected in series from anelectrical standpoint, n+2 elememts (n being a positive whole number)formed by an input line, n linear resonators each open at both ends andsubstantially of a given length ξb/2, and an output line, the resonatorsbeing arranged in the order of the first to the nth between respectivelythe input line and the output line, wherein a combination of n+1capacitive couplings are provided to respectively couple the input lineto the first end of the first resonator, the second ends of the i-thresonators to the first ends of the (i+1)th resonators (where i is awhole number from 1 to n-1, inclusive) and the second end of the nthresonator to the output line, and at least one pair of linear resonatorsof length λs/4 (λs being the wavelength to be filtered out and beingless than λb), the resonators of any given pair each having one endconnected to one of the n+2 elements and being spaced an electricaldistance (2k+1) λb/4 apart (where k is a whole number greater than -1).

The invention will be more readily understood, and others of itsfeatures be made apparent, in reading the following description withreference to the appended drawings in which:

FIG. 1 illustrates a first embodiment of a filter according to theinvention;

FIG. 2 shows the frequency response curves of the filter according toFIG. 1;

and FIGS. 3 through 5 illustrate a second embodiment of the invention.

Like parts are designated by like references in the different figures.

As can be seen by examining FIG. 1, the stripline filter according tothe invention comprises a substrate P of polytetrafluoroethylene glass,commercially known as Teflon Glass, in the form of a rectangular plate45 mm wide, 65 mm long and 1.6 mm thick. The hidden face of saidsubstrate P is entirely covered with copper deposited threon to serve asa ground plane; on the visible face, deposited copper strips A, 1through 7, 10, 11, 70, 71 and B form respectively an input line, sevenlinear resonators open at both ends, four auxiliary linear resonatorseach short-circuited at one end, and an output line. Attention is drawnto the fact that although the filters as described herein and in theaccompanying claims are described as having an input line such as A inFIG. 1 and an output line such as B in the same figure, the functions ofthese two lines could in fact be reversed such that line A serve as theoutput lead and line B as the input lead. The filter according to FIG. 1is of the type having striplines physically arranged in parallel.Indeed, the resonators 1 through 7 consist of line sections disposed inparallel to ensure compactness for the filter. The line sectionsdisposed between said input and output lines A and B. Resonators 1through 7 are halfwave conductive strips, all substantially λb/2 inlength, where λb is the wavelength corresponding to the center frequencyof the filter pass-band. To obtain a powerful coupling between thesuccessive stripline sections of the filter and ensure that thiscoupling is easily reproducible from one filter to the next inproduction, variable capacitors C0 to C7 are provided to linkrespectively the line A to the first end of the resonator 1, the secondend of resonator 1 to the first end of resonator 2 and so on up to thesecond end of resonator 6 and the first end of resonator 7 and thesecond end of resonator 7 to the line B; said line sections 1 through 7,together with the capacitors C1 through C6, thus form a zigzag pattern.

The four auxiliary resonators 10, 11, 70 and 71, which are quarterwavelines, are short-circuit connected by connecting one of their ends tothe corresponding main resonator, namely main resonator 1 for auxiliaryresonators 10 and 11, and main resonator 7 for auxiliary resonators 70and 71. These short-circuit resonators are designed to introduce a bandcutoff capability in the filter so that, as will be seen in FIG. 2, thefilter's amplitude/frequency response curve will have a steeper-edgedpass-band toward the higher frequencies. Accordingly, the length of saidauxiliary resonators is selected to be equal to λs/4 where λs is awavelength to be rejected, less than λb and substantially the same asthe center frequency of the frequency band to be excluded by the bandcutoff. The auxiliary resonators are associated in pairs, namely 10-11and 70-71 and the resonators of a pair are spaced apart a distance equalto (2 k+1)λb/4, with k a positive whole number set equal to 1 in theexample described. The choice of this spacing between the auxiliaryresonators affords a mutual compensation, in the filter's pass-band, ofthe inductive and capacitive disturbances introduced by each of theresonators in a same pair. Attention may also be drawn to the fact thatsaid auxiliary resonators associated with the notching or band cutofffunction can actually be located anywhere along the electrical pathbetween the two filter leads as long as the said distance between themof (2 k+1) λb/4 is maintained. The filter just described has a frequencypassband at 3 decibels of 950 to 1700 MHz, with a sharp attenuation to30 decibels on both sides of this band.

A graph of the amplitude/frequency response curves for the filterrepresented in FIG. 1 is given in FIG. 2, showing three curves G1, G2and G3.

Curve G1 represents the response of the circuit of FIG. 1 when providedwith high-valued coupling capacitors C0 to C7 (C0 and C7=20 pF and C1 toC6=5 pF) but lacking any auxiliary resonators 10, 11, 70, 71; this curveis substantially that of a high-pass filter providing an attenuation ofmore than 30 dB for frequencies below 200 MHz, changing from 30 dB to 1dB between 200 and 500 MHz, then of the order of 1 to 2 dB between 500and 1600 MHz (flat response) and thereafter varying from 1 to 11 dB forthe remainder of the frequencies covered by the measurement, ie. between1600 and 3750 MHz. This frequency response is far from corresponding tothe passband of the filter shown in FIG. 1, namely 950-1700 MHz.

Curve G2 of FIG. 2 represents the amplitude/frequency response of thecircuit of FIG. 1 without the auxiliary resonators 10, 11, 70, 71 butwith the capacitors set as in the inventive filter, as for G3 (C0, C7=15pF, C1 and C6=3 pF, C2 to C5=1.5 pF). The response is as desired for thelow frequencies but in the high frequencies the attenuation isinsufficiently sharp.

Curve G3 represents the amplitude/frequency response of the circuitshown in FIG. 1; comparing this curve with curve G2 makes it apparentthat adding the notch filter, providing a stop-band roughly centered on2300 MHz by means of the quarterwave lines the resonance frequencieswhereof are selected to lie in the 1850-2500 MHz band, has the effect ofbringing about a sharp change in attenuation in the neighborhood of thehigh frequecies of the bandpass filter: attenuation of less than 3 dBbelow 1750 MHz and of the order of 20 to 30 dB for frequencies rom 1800MHz to more than 2500 MHz; the attenuation is again lessened atfrequencies of the order of 2700 MHz and above, but the latterfrequencies are sufficiently removed from the filter bandwidth (950-1700MHz) to avoid any adverse effects in most applications of the filter.

Another embodiment of the filter according to the invention will now bedescribed with reference to FIGS. 3 through 5; this filter has in factthe same characteristics as the filter according to FIG. 1, but isfabricated with two conductive layers plus a ground plane on flexiblesubstrates and the capacitors thereof corresponding to the capacitors C1to C6 of FIG. 1 are obtained by overlapping the ends of lines which areseparated by the thickness of a flexible substrate.

FIG. 3 shows a flexible substrate of polyamide S1 on which six copperstrips have been deposited: A, 1+10 and 11, 3, 5, 7+70 and 71, and B.FIG. 4 shows another flexible polyamide substrate S2 on which threecopper strips have been deposited: 2, 4 and 6. The substrates S1 and S2are two 35×144 mm rectangular plates which are then glued one upon theother to produce the circuit assembly represented in FIG. 5. Also gluedbeneath the plates S1 and S2 is a ground plane consisting of a polyamidesubstrate one face whereof is coated with a cuprous deposit; this groundplane is not visible in FIG. 5.

To constitute a filter comparable to that of FIG. 1, it is onlynecessary to add to the assembly formed by the plates S1 and S2 andtheir deposition strips and ground plane two miniature fixed capacitorsrated at 15 picofarads each, designated C0 and C7 in FIG. 5. As in FIG.1, the input and output lines are labelled respectively A and B, thehalfwave line resonators are labelled 1 through 7 and the quarterwaveresonators are labelled 10, 11, 70 and 71. The capacitive couplingsbetween line A and resonator 1 and between resonator 7 and line B arerespectively realized by the capacitors C0 and C7. However, thecouplings between the halfwave resonators are obtained in this case byaligning the ends to be coupled; the facing surfaces, separated by thedielectric of the polyamide substrate, thus form the two plates of thecoupling capacitors; these capacitors bear the references C1 to C6 inFIG. 5.

Various other constructions of a band-pass filter are possible withoutdeparting from the scope of the invention. For instance, the filteraccording to the invention can be designed with a three-plate structure,in other words with the resonators disposed in the space separating twoparallel ground planes. Likewise, on the basis of the embodimentaccording to FIG. 1, the capacitors C1 to C7 can be made using metaltabs deposited onto a dielectric substrate; such tabs would be arrangedso that, to replace the capacitor C1 of FIG. 1 for example, the two endsof the tab align with the respective ends of the resonators 1 and 2 towhich said capacitor C1 was connected; the areas so aligned determinethe coupling between successive rsonators. Capacitors such as C0 and C7in FIGS. 1 and 5 can likewise be obtained by this technique of facingsurface areas of copper, or alternatively by the technique illustratedin FIG. 5 or equivalent. This is possible by giving the facing endslarge enough surface areas according to the thickness and thepermitivity of the dielectric separating them and the capacitancesought.

It is also possible to construct a filter comprising only a singlehalfway type resonator and a single pair of quarterwave resonators.

We claim:
 1. Band-pass filter for hyperfrequency electromagnetic waves,comprising, in series from an electrical standpoint, n+2 elements (nbeing a positive integer) formed by an input line, n linear resonatorsopen at both ends and all sustantially of a given length b/2, and anoutput line, the resonators being arranged in the order of the first tothe n-th between respectively the input line and the output line,wherein a combination of n+1 capacitors are provided to respectivelycouple the input line to the first end of the first resonator, thesecond ends of the i-th resonators to the first ends of the (i+1)thresonators (i being an integer from 1 to n-1, inclusive) and the secondend of the n-th resonator to the output line, and at least one pair ofauxiliary linear resonators of length λs/4 λs being the wavelength to berejected and less than λb) where b is the wavelength corresponding tothe center frequency of the filter pass-band, the auxiliary λb/4resonators of a pair, each pair having one end connected to one of then+2 elements and being spaced an electrical distance (2 k+1) apart (kbeing an integer greater than -1), and wherein the linear resonators arehalf wave conductive strips open at both ends and situated in paralleland along at least a part of their length which are colinear to create ahighly compact filter and permitting the auxiliary resonators to belocated anywhere along the electrical path between the two filter leadsas long as the distance therebetween is maintained at (2 K+1)λb/4. 2.Band-pass filter according to claim 1, wherein the n+2 elements areconstructed according to the stripline waveguide technique, said n+2elements being distributed on the two sides of a same dielectricsubstrate and at least one of the n+1 capacitors between two elementsbeing obtained by aligning one end of one of said elements with one endof the other of the two said elements, the two said elements beingdisposed for the purpose in opposite ends of the substrate and thesurface area of their facing ends being determined as a function of thecapacitance that is sought.